Field of the Invention
The present invention relates to a focus detection apparatus and a control method thereof and, more particularly, to a focus detection apparatus that performs phase-difference detection type focus detection based on an output from an image sensor, and a control method thereof.
Description of the Related Art
Japanese Patent Laid-Open No. 2008-52009 discloses a device that performs pupil-division type focus detection using an image sensor in which microlenses are formed in respective two-dimensionally arranged pixels. In this device, the photoelectric conversion unit of each pixel is divided into two areas where light fluxes having passed through different areas of the exit pupil of an imaging lens are received through the microlens. The phase difference (shift amount) between paired image signals is computed by performing correlation calculation on a pair of image signals generated from a plurality of pixels in every divided area. The defocus amount can be computed from the phase difference.
FIGS. 24 and 25 of Japanese Patent Laid-Open No. 2008-52009, and Japanese Patent No. 3592147 disclose image sensors in which some pixels are set as focus detection pixels for performing pupil-division type focus detection. Correction is necessary to use an output from the focus detection pixel as an output from a normal pixel. However, the number of signals read out as focus detection signals is smaller than in an arrangement in which the photoelectric conversion unit of each pixel is divided. Thus, the manufacturing cost of the image sensor and the calculation cost of signal processing can be suppressed.
In an arrangement using focus detection pixels, paired photoelectric conversion units that receive, via microlenses, light fluxes having passed through different areas of the exit pupil of an imaging lens are arranged in different pixels. That is, the positions of pixel groups used to generate a pair of image signals (A image and B image) for detecting a phase difference are different between the A image and the B image. Depending on the pattern of an object image, the degree of similarity between the A image and the B image may be low. In such a case, the focus detection accuracy degrades.
When the arrangement interval between focus detection pixels is large, the frequency component of the high-frequency band of an object optical image may not be acquired. Different folding noise components are generated in the A image and the B image, generating a focus detection error.
In an arrangement in which a plurality of photoelectric conversion units are arranged in each pixel, addition for generating a normal image signal is necessary in addition to readout from each photoelectric conversion unit, increasing the load of readout processing and calculation processing. The load may be reduced by performing readout of an output from one photoelectric conversion unit and addition readout of outputs from the two photoelectric conversion units, and generating an output from the other photoelectric conversion unit by subtraction. However, the subtraction generates an error, and there is a fear of degrading the phase difference detection accuracy.